Military platforms and installations must be able to operate effectively when under attack or threat of attack by NBC weapons. To ensure that operating efficiency is maintained, collective protection is normally provided by creating a clean positive pressure environment within the enclosed occupied space with filtered air. Under such conditions personnel can continue to operate without the encumbrance of protective gloves and respirators which greatly reduce operating effectiveness.
Traditionally, environmental life support systems have been based upon activated charcoal filters for gas filtration, cooling being by vapour cycle refrigeration. Although providing a high level of protection, activated charcoal filters are not regenerative and must be discarded and replaced following a chemical attack. The effective life of the filters is dependent on the concentration and nature of the challenge but may only be a few hours with some chemicals.
This severely limits the effective operation of a platform when chemical weapons are deployed against them and creates high operational costs due to the logistics chain required to provide a regular supply of new filters and also the concomitant removal and disposal of contaminated filters.
More recently systems have been developed based upon regenerative filtration in place of the non-reusable activated carbon elements.
A number of proposals have been made to provide systems utilising pressure swing adsorption (PSA) systems developed by PALL Corporation.
Pressure swing adsorption systems use beds filled with a sorbent material which adsorbs gases under pressure and desorbs gases once the pressure is removed. The system has two beds, one on-line and fed with contaminated air under pressure from which contaminants are removed. At the same time, the other depressurised off-line bed is regenerating by purging with clean air. On completion of the cycle, the role of each bed is reversed, the operation being controlled by an automatic sequence timer to provide continuous uninterrupted service. Industrial PSA systems have been shown to operate for many years without degradation of performance or air quality. PSA systems developed by PALL Corporation have been shown to be efficient in removing a very wide range of chemical agents.
A number of proposals for environmental life support systems using PSA have appeared, as a consequence, in the patent literature.
U.S. Pat. No. 4,732,579, Veltman et al assigned to FMC Corporation, the disclosures of which are incorporated herein, proposes a system and method for providing a continuous supply of clean air at a desired temperature to the crew members of a combat vehicle. The contaminated air is said to be initially compressed by energy received from the exhaust gases from a combustion power unit of the vehicle, the initially compressed air being cooled to increase its density and then compressed and cooled a second time before being passed through a pressure swing adsorbent system. Air from the PSA system is expanded and changed in temperature to provide clean air to personnel within the vehicle. Energy released from the air during expansion is used to compress the air in the secondary compressor. The off line PSA bed is purged with clean air from the on line bed which is expanded through an orifice to lower its pressure.
U.S. Pat. No. 4,769,051, Defranceso assigned to United Technologies Corporation, the disclosures of which are incorporated herein, discloses an air conditioning system powered by a supply of compressed air. The compressed air passes to an air cycle machine having a compressor, a turbine and a load heat exchanger. Air from the compressor is communicated to the turbine which expands and cools the air before passing it to the load heat exchanger. A PSA system cleans air as it passes from the compressor to the turbine before expansion. Purge air for the PSA system is derived from the clean air exiting the load heat exchanger after first passing it through a regenerative heat exchanger which takes heat from the air as it passes from the compressor to the PSA system before passing on to the turbine.
U.S. Pat. No. 5,213,593, White et al assigned to PALL Corporation, the disclosures of which are incorporated herein, proposes a PSA system which has first and second sorbing chambers, each of which includes first and second openings defining a gas flow path between them and a sorbent bed disposed in the gas flow path and having a sorption inlet region near the first opening, and a heater positioned near the sorption inlet region. The heaters are operated from an external energy source. A valve arrangement interconnects an intake, an exhaust and the first openings of the first and second sorbing chambers and interconnects an outlet with the second openings of the first and second sorbing chambers. Gas is directed through one sorbing chamber to the outlet. At the same time a portion of the outlet gas is directed through the other sorbing chamber to the exhaust. Energy from the external energy source is coupled to the heater of the other sorbing chamber to heat the sorption inlet region of the other sorbing chamber as the outlet gas flows through the sorption inlet region. The controller is adapted to cycle between the first and second sorbing chambers according to a NEMA cycle length of less than about five minutes. One sorbent bed adsorbs at least a portion of contaminant from the gas and is heated by the heat of adsorption and the other sorbent bed is regenerated using both the energy supplied by the heater and the heat of adsorption.
A problem with previously proposed PSA systems is the high energy requirement for the compression of the contaminated air required to achieve the pressures required for the PSA beds. This energy requirement has been a constraint on the commercialisation of PSA systems. Commonly owned earlier U.S. Pat. No. 6,099,617, Bennett, the disclosures of which are incorporated herein, discloses a method of providing clean air at a desired temperature to an environment which is more energy efficient than prior systems by using heat derived from the step of compressing the incoming air elsewhere in the system. The method comprises compressing incoming air possibly contaminated with nuclear, biological and chemical contaminants, cooling the compressed air in a first heat exchanger, compressing the cooled air from the first heat exchanger in a secondary compressor, cooling the compressed air from the secondary compressor in a secondary heat exchanger, and directing the cooled air from the secondary heat exchanger through a regenerative pressure swing adsorption system for providing clean output air with the contaminants removed therefrom. The cleaned air is then expanded in a turbine coupled to the secondary compressor, thereby recovering energy from the cleaned air to drive the secondary compressor. The expanded cleaned air from the turbine is used to condition air in the environment and to purge the regenerative pressure swing adsorption system. The purging air is warmed before being passed to the regenerative pressure swing adsorption system by heat derived from the step of compressing incoming air, for example by heat exchange with cleaned air exiting the regenerative pressure swing adsorption system, to pass purging air to the regenerative pressure swing adsorption system at the optimum temperature for efficient operation thereof.
Whilst the system described in our earlier US Patent makes use of waste energy within the system, and therefore improves the efficiency of the system, the overall life support system is still considered energy inefficient.